The present invention relates to a series of new groups of radicalic polymerization initiators of unsaturated vinyl and/or vinylidene monomers.
More specifically, the present invention relates to a new group of initiators for vinylaromatic monomers and their use in the radicalic polymerization thereof.
The initiators object of the present invention give the polymerization the characteristics of a xe2x80x9clivingxe2x80x9d polymerization and therefore allow the production of block copolymers. With respect to the classical systems based on nitroxyl and peroxide radicals, they activate the polymerization of styrene at much lower temperatures (about 100xc2x0 C.).
The polymerization of unsaturated monomers proceeds radicalically in the presence of suitable initiators, normally represented by peroxides (for example benzoyl peroxide, dicumyl peroxide, etc.) or by azo-compounds such as, for example, azobis(isobutyronitrile). In some cases, as in the case of styrene for example, the polymerization can be effected spontaneously by heating the monomer to over a certain temperature (100-110xc2x0 C.), in correspondence with which there is the formation of particular adducts containing unpaired electrons which start the polymerization. In all these cases the polymerization is xe2x80x9cnon-livingxe2x80x9d, i.e. the polymeric macroradical increases its molecular weight in a very short time and undergoes end or transfer reactions which cause the interruption of the chain. Other chains begin to form contemporaneously, due to the reaction with the initiator, which is characterized by its own half-life time and consequently continuously generates the radicalic species responsible for the polymerization over a period of time.
The result of this process is that it is impossible to control the molecular weights and, as a result of the end and transfer reactions, it is not possible to prepare block polymers, as is the case, on the contrary, with anionic polymerization. In this latter type of polymerization, called xe2x80x9clivingxe2x80x9d, there are practically no transfer or end reactions, and it is therefore possible to induce the growth of a block of a second monomer on a macromolecule. The polymeric chains, furthermore, all begin contemporaneously and grow at the same rate so that the end polymer has a very narrow molecular weight distribution and the molecular weight is exclusively determined by the monomer/initiator ratio, which can be pre-established as desired.
A series of initiator systems has recently been found, which are capable of also giving radicalic polymerization the characteristics of a xe2x80x9clivingxe2x80x9d process. The use of iniferters is described in xe2x80x9cMacromolecular Chemistry, Rapid Communicationxe2x80x9d 3, 127, 1982. These substances act as thermal and/or photochemical initiators as well as transfer agents and reversible chain-terminators; if they were not reversible they would fall into the category of classical radicalic polymerization. The iniferters specified are di-alkylthiouram disulfides, diaryl disulfides, etc. Monomers selected from methylmethacrylate, styrene, methylacrylate and vinylacetate are polymerized.
The disadvantages of this technique lie in the fact that both of the radicals produced by the decomposition of the initiator are capable of adding monomers and that there is limited industrial applicability owing to the use, in most cases, of UV radiation to initiate the process; in addition to this there are significant chain-termination reactions with a consequent loss in the polymerization life, as described in xe2x80x9cPolymer Bulletinxe2x80x9d (Berlin), 7, 197 1982).
Other examples of initiators are tetra-arylethanes which thermally decompose to give diphenyl alkyl radicals (Macromolecular Chemistry, 184, 745, 1983) and silylated pinacols (Journal of Polymer Science, Polymer Chemistry Ed, 24, 1251, 1986), but these systems are not very efficient and have therefore never been developed.
In U.S. Pat. No. 4,581,429 there is a first reference to the synthesis of homo and copolymers by means of the use of initiators of the R1R2Nxe2x80x94Oxe2x80x94X type (alkoxyamines) wherein R1 and R2 are substituents with no hydrogen in the carbon adjacent to the nitrogen atom, whereas X is a substituent of such a nature that the corresponding X radical, formed as a result of the thermal breakage of the NO-X bond, is capable of polymerizing unsaturated monomers by means of a radicalic mechanism. The chain growth control is due to the fact that the breakage reaction of the bond is an equilibrium reaction, and the nitroxy-radical formed is not capable of initiating the radicalic polymerization of the monomer. The use of alkoxyamines variously substituted and their synthesis is also described in Macromolecules, 28, 2993 (1995) and in Polymer Preprints, 40, 2, 315 (1999).
In U.S. Pat. No. 5,322,912, the alkoxyamine is generated directly in the reaction environment by mixing the stable nitroxy-radical, the peroxide and the monomer and heating to a suitable temperature.
U.S. Pat. Nos. 5,627,248 and 5,677,388 describe the use of bifunctional alkoxyamines having general formula R4R5Nxe2x80x94Oxe2x80x94C(R2R3)xe2x80x94R1xe2x80x94C(R2R3)xe2x80x94Oxe2x80x94NR4R5 in the radicalic polymerization process.
U.S. Pat. No. 5,910,549 describes a method for the preparation of alkoxyamines starting from nitroxy-radicals but in this, as in all previous cases, in the claims relating to the possible nitroxy-radicals or possible alkoxyamines, nitrogen and oxygen never form part of a cycle.
In international patent application WO 96/30421 a new process is proposed, consisting in the addition of a monomer to the growing radical generated by an alkyl halide by means of a reversible redox reaction catalyzed by transition metals such as Cu(I)/2bipyridyl. Polar monomers can be polymerized in this way, with the possibility of also obtaining block and grafted copolymers. One of the disadvantages of this technology is associated with the metallic residue in the synthesized material which can cause degradation of the chains undergoing transformation, and also the production of low molecular weights.
Polym. Prep., 35(1), 704 (1994) describes the use of cobalt porphyrins as controllers in the polymerization of methacrylates; although these systems produce polymers with high molecular weights and a low polydispersity, they have a high cost and, if not supported and therefore filtered, give the polymer an undesired colouring.
International patent application WO 98/01478 describes a new living radicalic polymerization method called RAFT (Reversible Addition-Fragmentation Transfer) in which thio-esters having general formula Sxe2x95x90C(Z)SR are used as transfer agents. Acrylic monomers are also polymerized with this technique, but their release may cause problems relating to bad smell and undesired colouring of the polymer, owing to the low molecular weight of the sulfurized compounds.
The Applicant has now identified a new category of initiators active in the polymerization of unsaturated monomers, in particular vinylaromatic monomers, which have the additional advantage of allowing the formation of block structures. These initiators are already active at temperatures of 100xc2x0 C. and are thermally activated without having to resort to the use of particular radiation sources, which distinguishes them from previously known systems. Furthermore, unlike the systems based on the combination of peroxides or azo-compounds with nitroxyl radicals, they are xe2x80x9cmonocomponentxe2x80x9d, which greatly facilitates dosage in the reaction phase.
An object of the present invention therefore relates to organic initiators for the polymerization of unsaturated vinyl and/or vinylidene monomers characterized by the presence of a heterocyclic structure having a nitrogen atom bound to an oxygen atom in the same cycle and having the general formula selected from structures (I)-(X) illustrated below. 
wherein only one of R1, R2, R3, R4, R5 is hydrogen, whereas the remaining are a linear or branched C1-C6 alkyl radical, or C6-C12 aryl radical, if one of R4 or R5 is hydrogen then the remaining R4 or R5 is an aryl radical, if one of R1 or R2 or R3 is hydrogen then one and only one of the remaining R1 or R2 or R3 is an aryl radical; R6 represents a hydrogen atom or a linear or branched C1-C6 alkyl radical, or a xe2x80x94CH2xe2x80x94R14 group, wherein R14 represents a C1-C6 alkyl radical, C6-C12 aryl or C7-C15 alkylaryl radical; Ar is a phenyl which can contain substituents on the aromatic ring represented by halogens, linear or branched C1-C6 alkyl groups, carboxyl groups; R7-R13 independently represent a halogen, such as chlorine, or a hydrogen atom or are selected from C1-C6 alkyl groups, optionally halogenated, C6-C12 aryl groups, carboxyl, alkoxyl or acyl groups containing from 1 to 15 carbon atoms, sulfonic, phosphonic, phosphinic, amine, amide, nitric groups containing up to 15 carbon atoms.
Examples of products according to the group having general formula (I) are:
2-(1,1-dimethylethyl)-3-ethyl-4-phenyl-1,2 oxazethidine;
2-(1,1-dimethylethyl)-3-methyl-4-phenyl-1,2 oxazethidine;
2-(1,1-dimethylethyl)-3,3-dimethyl-4-phenyl-1,2oxazethidine;
2-(1,1-dimethylethyl)-3,4-diphenyl-1,2 oxazethidine.
Examples of products according to group (II) are:
2-(1,1-dimethylethyl)-3,3-dimethyl-5-phenyl-isoxazolidine;
2-(1,1-dimethylethyl)-3,5-diphenyl-isoxazolidine;
2-(1-methylethyl)-3,3-dimethyl-5-phenyl-isoxazolidine;
2-(1,1-dimethylethyl)-3,3-dimethyl-5(4-methoxyphenyl)-isoxazolidine;
2-(1,1-dimethylethyl)-3,3-dimethyl-5(4-chlorophenyl)-isoxazolidine;
2-(1,1-dimethylethyl)-3,3-dimethyl-5(2,4-dimethoxyphenyl)-isoxazolidine.
Examples of products according to group (III) are:
2-(1,1-dimethylethyl)-3,3-dimethyl-6-phenyl-2H-3,4-dihydro-5,6-dihydro-1,2-oxazine;
2-(1,1-dimethylethyl)-3,6-diphenyl-2H-3,4-dihydro-5,6-dihydro-1,2-oxazine;
2-(1-methylethyl)-3,3-dimethyl-6-phenyl-2H-3,4-dihydro-5,6-dihydro-1,2-oxazine;
2-(1,1-dimethylethyl)-3,3-dimethyl-6(4-methoxyphenyl)-2H-3,4-dihydro-5,6-dihydro-1,2-oxazine;
2-(1,1-dimethylethyl)-3,3-dimethyl-6(2,4-dimethoxyphenyl)-2H-3,4-dihydro-5,6-dihydro-1,2-oxazine;
2-(1,1-dimethylethyl)-3,3-dimethyl-6(4-chlorophenyl)-2H-3,4-dihydro-5,6-dihydro-1,2-oxazine.
Examples of products according to group (IV) are:
1-(1,1-dimethylethyl)-3-phenyl-1H-3,4-dihydro-2,1-benzoxazine;
1-(1,1-dimethylethyl)-3-phenyl-5-methyl-1H-3,4-dihydro-2,1-benzoxazine;
1-(1,1-dimethylethyl)-3-phenyl-5,8-dimethoxy-1H-3,4-dihydro-2,1-benzoxazine;
1-(1,1-dimethylethyl)-3-phenyl-6,7-dimethoxy-1H-3,4-dihydro-2,1-benzoxazine;
1-(1,1-dimethylethyl)-3-phenyl-5,8-dichloro-1H-3,4-dihydro-2,1-benzoxazine.
An example of a product having general formula (V) is:
1,2,3,4-tetrahydro-7H-11bH-pyrido[2,1-d][2,3]benzoxazine.
Examples of products according to group (VI) are:
1-(1,1-dimethylethyl)-3-ethyl-1H-3H-2,1-benzoxazole;
1-(1,1-dimethylethyl)-3-methyl-5-methoxy-1H-3H-2,1-benzoxazole;
1-(1,1-dimethylethyl)-3-propyl-4,7-dimethyl-1H-3H-2,1-benzoxazole;
1-(1-methylethyl)-3-ethyl-1H-3H-2,1-benzoxazole.
Examples of products according to group (VII) are:
1,4,4-trimethyl-3(1,1-dimethylethyl)-1H-3,4-dihydro-2,3-benzoxazine;
1,4,4-trimethyl-3(1-methylethyl)-1H-3,4-dihydro-2,3-benzoxazine;
1,4,4-trimethyl-3(1,1-dimethylethyl)-5-methoxy-1H-3,4-dihydro-2,3-benzoxazine;
1,4,4-trimethyl-3(1,1-dimethylethyl)-5,8-dichloro-1H-3,4-dihydro-2,3-benzoxazine;
An example of a product having general formula (VIII) is:
1-(1,1dimethylethyl)-1-aza-2-oxa-3H-phentalene.
Examples of products according to group (IX) are:
3-phenyl-2-oxa-6,6-dimethyl-1-azabicyclo [2.2.1] heptane;
3(4-methoxyphenyl)-2-oxa-6,6-dimethyl-1-azabicyclo [2.2.1] heptane;
3,6-diphenyl-2-oxa-6,6-dimethyl-1-azabicyclo [2.2.1] heptane;
3-phenyl-2-oxa-6,6-diethyl-1-azabicyclo [2.2.1] heptane;
Examples of products according to group (X) are:
3-phenyl-2-oxa-6,6-dimethyl-1-azabicyclo [2.2.2] octane;
3(4-methoxyphenyl)-2-oxa-6,6-dimethyl-1-azabicyclo [2.2.2] octane;
3,6-diphenyl-2-oxa-6,6-dimethyl-1-azabicyclo [2.2.2] octane;
3-phenyl-2-oxa-6,6-diethyl-1-azabicyclo [2.2.2] octane.
A further object of the present invention relates to a process for the polymerization of vinylaromatic monomers which comprises reacting at least one vinylaromatic monomer in the presence of one or more initiators having general formulae (I)-(X).
The term vinylaromatic monomers as used in the present description and claims mainly refers to styrene but can also refer to other styrene monomers having one or more hydrogen atoms substituted with C1-C4 alkyl or aryl radicals, a halogen or a nitro radical such as, for example, methyl-styrene, vinylnaphthalene, mono-, di-, tri-, tetra-, penta-chloro styrene, styrenes alkylated in the nucleus such as ortho-meta- and para-methylstyrene, ortho-meta- and para-ethylstyrene, etc., either alone or mixed with each other and/or with styrene.
The vinylaromatic monomer can be used in a mixture with an ethylenically unsaturated nitrile such as acrylonitrile or methacrylonitrile, for example in a quantity ranging from 0.1 to 50% by weight with respect to the total monomers, or, as an alternative to or in addition to ethylenically unsaturated nitrile, mixed with other ethylenically unsaturated monomers in such quantities that the vinylaromatic monomer is present in a concentration higher than 40% by weight.
Examples of ethylenically unsaturated monomers are alkyl or cycloalkyl esters of acrylic or methacrylic acid in which the alkyl or cycloalkyl groups contain from 1 to 4 carbon atoms and from 4 to 10 carbon atoms respectively, such as methylacrylate, methylmethacrylate, ethylacrylate, ethylmethacrylate, butylmethacrylate, cyclohexylmethacrylate, etc. Another ethylenically unsaturated monomer can be maleic anhydride.
Block copolymers can also be prepared by the polymerization of the first monomer or mixture of monomers up to a conversion ranging from 5 to 99% and subsequent feeding of the second monomer or mixture of monomers. The first copolymer block can be isolated by precipitation in a non-solvent and subsequently re-copolymerized by dissolution in the monomer or mixture of monomers forming the second copolymer block.
An inert solvent, which acts as diluent, is added to the mixture to be polymerized in a quantity not higher than 20% and preferably from 1 to 15% by weight, with respect to the mixture to be polymerized. Examples of suitable inert solvents are aromatic hydrocarbons such as ethylbenzene, ketones, esters and nitrites which are liquid at the polymerization temperature. In addition to the ethylbenzene mentioned above, toluene, xylenes or their mixtures, can be used, as aromatic hydrocarbons. Examples of ketones are 2-butanone, methylethylketone, cyclohexanone, etc. Other examples of solvents particularly suitable for the present process are ethyl acetate and acetonitrile.
The polymerization reaction is substantially carried out under the same conditions as the traditional peroxide polymerization, except for the reaction temperature which ranges from 100 to 130xc2x0 C., preferably below 120xc2x0 C. The polymerization can be carried out in the presence of water.
Some illustrative but non-limiting examples are provided for a better understanding of the present invention and for its embodiment.